<template>
  <scroll
    class="listview"
    :data="data"
    ref="listview"
    :listenScroll="listenScroll"
    @scroll="scroll"
    :probeType="probeType"
  >
    <ul>
      <li v-for="group in data" class="list-group" ref="listGroup">
        <h2 class="list-group-title">{{ group.title }}</h2>
        <ul>
          <li @click="selectItem(item)" v-for="item in group.items" class="list-group-item">
            <img v-lazy="item.avatar" class="avatar">
            <span class="name">{{ item.name }}</span>
          </li>
        </ul>
      </li>
    </ul>

    <div
      class="list-shortcut"
      @touchstart.stop.prevent="onShortcutTouchStart"
      @touchmove.stop.prevent="onShortcutTouchMove"
    >
      <ul>
        <li
          v-for="(item,index) in shortcutlist"
          class="item"
          :data-index="index"
          :class="{current: currentYIndex == index}"
        >{{ item }}</li>
      </ul>
    </div>

    <div class="list-fixed" v-show="fixedTitle" ref="fixed">
      <div class="fixed-title">{{ fixedTitle }}</div>
    </div>

    <div v-show="!data.length" class="loading-container">
      <loading></loading>
    </div>
  </scroll>
</template>

<script type="text/ecmascript-6">
import Scroll from "base/scroll";
import Loading from "base/loading";
import { getData } from "common/js/dom";

const SHOT_HEIGHT = 18;
const TITLE_HEIGHT = 30;

export default {
  name: "ListView",
  props: {
    data: {
      type: Array,
      default: []
    }
  },
  data() {
    return {
      scrollY: -1, //  监测实时滚动的位置
      currentYIndex: 0,
      diff: -1 //  吸顶的偏移量
    };
  },
  computed: {
    shortcutlist() {
      return this.data.map(i => {
        return i.title.substring(0, 1);
      });
    },
    fixedTitle() {
      if (this.scrollY > 0) return "";
      return this.data[this.currentYIndex]
        ? this.data[this.currentYIndex].title
        : "";
    }
  },
  watch: {
    data() {
      setTimeout(() => {
        this._groupHeight();
      }, 20);
    },
    scrollY(newY) {
      // 判断滚动的高度在数组中的哪个区间内
      const listHeight = this.listHeight;

      // 当滚动到顶部 newY>0
      if (newY > 0) {
        this.currentYIndex = 0;
        return;
      }
      // 在中间滚动
      for (let i = 0; i < listHeight.length - 1; i++) {
        let height1 = listHeight[i];
        let height2 = listHeight[i + 1];
        if (-newY >= height1 && -newY < height2) {
          this.currentYIndex = i;
          this.diff = height2 + newY;
          return;
        }
      }
      // 滚动到底部，且 -newY 大于最后一个元素的上限
      this.currentYIndex = listHeight.length - 2;
    },
    diff(newVal) {
      // 吸顶偏移量
      let fixedTop =
        newVal > 0 && newVal < TITLE_HEIGHT ? newVal - TITLE_HEIGHT : 0;

      if (this.fixedTop === fixedTop) return; //  减少dom赋值的操作频率   之前都为0
      this.fixedTop = fixedTop;
      this.$refs.fixed.style.transform = `translate3d(0, ${fixedTop}px, 0)`;
    }
  },
  components: {
    Scroll,
    Loading
  },

  created() {
    // 不需要实时监测的数据可以放在created上，并直接定义到this上
    this.touch = {};
    this.listenScroll = true;
    this.listHeight = [];
    this.probeType = 3; //  不截流  实时监听
  },

  methods: {
    selectItem(item) {
      // 将点击的元素派发出去
      this.$emit('select', item)
    },
    onShortcutTouchStart(e) {
      // 获取元素上的 data-index
      let currentIndex = getData(e.target, "index");
      this.touch.y1 = e.touches[0].pageY; //  y 轴位置
      this.touch.shotIndex = currentIndex;
      this._scrollTo(currentIndex);
    },
    onShortcutTouchMove(e) {
      this.touch.y2 = e.touches[0].pageY; //  y 轴位置
      // 获取 index
      let index =
        (((this.touch.y2 - this.touch.y1) / SHOT_HEIGHT) | 0) +
        (this.touch.shotIndex - 0);
      this._scrollTo(index);
    },
    scroll(pos) {
      // 监听滚动
      this.scrollY = pos.y;
    },

    _scrollTo(currentIndex) {
      if (!currentIndex) return;
      if (currentIndex < 0) {
        currentIndex = 0;
      } else if (currentIndex > this.listHeight.length - 2) {
        currentIndex = this.listHeight.length - 2;
      }
      this.scrollY = -this.listHeight[currentIndex];
      this.$refs.listview.scrollToElement(
        this.$refs.listGroup[currentIndex],
        0
      );
    },
    _groupHeight() {
      // 获取每个元素的高度
      this.listHeight = [];
      const list = this.$refs.listGroup;
      let height = 0;
      this.listHeight.push(height);
      for (let i = 0; i < list.length; i++) {
        let item = list[i];
        height += item.clientHeight;
        this.listHeight.push(height);
      }
    }
  }
};
</script>

<style lang="stylus" scoped>
@import '~common/stylus/variable';

.listview {
  position: relative;
  width: 100%;
  height: 100%;
  overflow: hidden;
  background: $color-background;

  .list-group {
    padding-bottom: 30px;

    .list-group-title {
      height: 30px;
      line-height: 30px;
      padding-left: 20px;
      font-size: $font-size-small;
      color: $color-text-l;
      background: $color-highlight-background;
    }

    .list-group-item {
      display: flex;
      align-items: center;
      padding: 20px 0 0 30px;

      .avatar {
        width: 50px;
        height: 50px;
        border-radius: 50%;
      }

      .name {
        margin-left: 20px;
        color: $color-text-l;
        font-size: $font-size-medium;
      }
    }
  }

  .list-shortcut {
    position: absolute;
    z-index: 30;
    right: 0;
    top: 50%;
    transform: translateY(-50%);
    width: 20px;
    padding: 20px 0;
    border-radius: 10px;
    text-align: center;
    background: $color-background-d;
    font-family: Helvetica;

    .item {
      padding: 3px;
      line-height: 1;
      color: $color-text-l;
      font-size: $font-size-small;

      &.current {
        color: $color-theme;
      }
    }
  }

  .list-fixed {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;

    .fixed-title {
      height: 30px;
      line-height: 30px;
      padding-left: 20px;
      font-size: $font-size-small;
      color: $color-text-l;
      background: $color-highlight-background;
    }
  }

  .loading-container {
    position: absolute;
    width: 100%;
    top: 50%;
    transform: translateY(-50%);
  }
}
</style>
